The Glitch
Overview: An installer is asked if a geothermal heat pump can
be combined with radiant floor heating. He responds with, “Of course, it’s just
a matter of substituting the heat pump for a boiler.” The system he creates is
shown below.
It’s meant to supply several independently controlled zones of floor heating.
The geothermal water-to-water heat pump is a single-speed unit with a rated output
of 60,000 Btu/hr. It has plenty of capacity to meet the design heating load of
the building.
Exercise: So what’s wrong with this system design?
The Fix
The heat pump will respond
just like a fixed capacity boiler. Whenever there is a call for heat from a
zone, it will turn on. In many cases it will generate heat at a rate far
greater than the rate of heat dissipation by the active zone(s). Since there is
very little thermal mass in the system, it will short cycle.
This is especially hard on the compressor. The starting amperage of a compressor
this large will also tend to momentarily dim the lights (even with a 200 amp service
entrance).
The system needs thermal mass in the form of a well-insulated buffer tank. It
also should have an expansion tank and air eliminator on the earth loop.
Because of the extensive zoning using valve actuators the distribution system
should have a differential pressure bypass valve, or even better, a variable
speed pressure regulated circulator. The temperature in the buffer tank can be
regulated with outdoor reset control in response to outdoor temperature. This allows
the heat pump to operate with the lowest possible supply water temperature and
thus the highest possible efficiency.
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